The present invention relates to a liquid crystal display apparatus of active matrix type and a driving method therefor.
FIG. 13 is a diagram of a driving circuit of a conventional active matrix type liquid crystal display apparatus as described in Japanese Unexamined Patent Publication No. 313607/1993. In this diagram, a plurality of X electrode lines (Xixe2x88x921, Xi, Xi+1. . . ) and Y electrode lines (Yjxe2x88x921, Yj, Yj+1 . . . ) are arranged in a form of a matrix, and active elements 11 and liquid crystal elements 12 such as TFT (thin film transistors) are formed on intersections of each of the X electrode lines and Y electrode lines. The Y electrode lines are also called data lines and are connected to a display signal circuit 13 for outputting display data signals for each of the liquid crystal display elements 12. Further, the X electrode lines are also called scanning signal lines and are connected to a scanning signal circuit 14 for outputting scanning signals.
The counter side of the liquid crystal display elements 12 are connected to a common electrode 15. The driving of the active elements 11 is performed in that the active elements 11 on the X electrode lines are set to at ON conditions (active conditions) synchronously with the scanning of the X electrode lines, in that display data signals are output from the display signal circuit 13, and in that data signals are written into corresponding liquid crystal display elements 12 via the active elements 11 in ON conditions. It should be noted that there have also been taken measures in which storage capacitances 16 are provided, upon requirement, for the liquid crystal display elements 12 in order to improve storage characteristics of electrical charge of the liquid crystal display elements 12.
An example of a conventional driving method for an active matrix type liquid crystal display apparatus is disclosed in Japanese Unexamined Patent Publication No. 141269/1994, and FIG. 14 is a view showing an example of a timing chart for indicating the driving method. As known, liquid crystal need to be driven through alternating-current, whereby an electric potential 111 of the signal line is made to be an image signal performing alternating-current inversion with a certain electric potential Vc being the center. During vertical scanning period T1, an electric potential 112 of a scanning line becomes high-leveled by a single scanning period T3. Such a scanning pulse is sequentially applied from above the screen per scanning line. T2 denotes a vertical blanking period (hereinafter also referred to as mere xe2x80x9cblanking periodxe2x80x9d) in which usually no image signals are applied. An electric potential of a counter electrode 113 is set to be lower than the central electric potential Vc of the image signal in case of N channel TFTs.
It will now be explained for a line common inversion driving method that is one of the objects of the present invention as a driving method for the above liquid crystal display apparatus. In a line common inversion driving method, two adjacent pixels are driven through alternating-current to be of opposite polarity, and this method is advantaged in that a driving IC of low cost may be employed and the power consumption can be decreased.
Driving waveforms of a conventional TFT-LCD employing the line common inversion method is shown in FIG. 15 and FIG. 16. FIG. 15 is a view of driving waveforms of odd-numbered lines and FIG. 16 of driving waveforms of even-numbered lines, respectively. In FIGS. 15 and 16, Vd denotes electric potentials of drain electrodes (broken line of short pitches), Vcom electric potentials of the counter electrodes (thin real line), Veff voltage applied onto the liquid crystal (potential differences between Vd and Vcom are shown by the hatching), Vg electric potentials of the gate lines including voltage at the time of gate OFF Vgl and voltage at the time of gate ON Vgh. Vs denotes electric potentials of source lines (broken line of long pitches). In case the reference marks Vcom, Vg, Vs are indicated in connection with the word xe2x80x9csignalxe2x80x9d such as xe2x80x9cVcom signalxe2x80x9d, these represent signals having electric potentials of counter electrodes. Further, DA denotes data period, and BK blanking period, respectively. The effective voltage Veff that is applied on the liquid crystal corresponds to a root-mean-square of a single frame period of an electric potential difference between Vd and Vcom. The Vd varies per Single Horizontal period (1H) depending on the Vcom, Vg and Vs signals.
In alternating-current driving based on a line common inversion method, Vg is controlled by the scanning signal circuit, Vs by the display signal circuit, Vcom by a timing control circuit and a power source circuit (not shown), while Vd is determined by the Vg, Vs and Vcom. A Single Horizontal period (1H) is approximately 32 xcexcs in case of VGA, approximately 26 xcexcs in case of SVGA, and 20 xcexcs in case of XGA, and a value for each of the electric potentials Vgh is set to be a voltage with which charge/discharge of electric charge of the drain electrodes can be completed within 1H, that for Vgl to be a voltage with which the electric charge of the drain electrodes can be sufficiently held during a single frame period, and those for Vs and Vcom to be a voltage with which display can be performed at a desired luminance.
Since this variation is repeated per 1H during the data display period, the Veff of the odd-numbered lines and even-numbered lines can be set to be identical by optimizing the central value for the Vcom. However, since the Vcom, Vgl and Vs signal are usually not varied but remain fixed during the blanking period, the drain variation at the start of the blanking period is maintained during the blanking period whereby luminance differences are generated line by line owing to the different values for the Veff of the odd-numbered lines and even-numbered lines during the blanking period as shown in FIG. 15 and FIG. 16. The relationship between Vd and Vcom will now be explained. As shown in FIG. 17, the electric potential Vd of the pixel electrodes (drain electrodes) changes owing to effects of variations in (1) electric potential Vcom of the counter electrodes 22, (2) electric potential of a storage electrodes 23, and (3) electric potential Vs of the source electrodes in a holding condition of the TFTS: 21. However, the electric potential of the storage electrode is determined by the Vg, Vcom and other factors, and a signal identical in amplitude and polarity with those of the Vcom (while the DC values may be different) is applied. In this manner, since the alternation of (1) to (3) is terminated during the blanking period, it may happen that the luminance differences of brightness between odd-numbered lines and even-numbered lines are generated.
The present invention has been made to solve such problems, and it is an object thereof to provide a liquid crystal display apparatus and a driving method therefor in which TFT driving signals during blanking periods are optimized to compensate for effective voltage differences during vertical blanking periods between odd-numbered lines and even-numbered lines and to decrease luminance differences per gate line.
In order to compensate for effective voltage differences during the afore-mentioned vertical blanking period, the present invention employs a means to perform alternation of the afore-mentioned (1) electric potential Vcom of counter electrodes 22, (2) electric potential of storage electrodes 23, and (3) electric potential Vs of source electrodes also during the blanking period as it is similarly performed during the data period. However, it is also possible to perform alternation of only one of (1), (2) and (3) during the blanking period or to combine some of these. It will now be shown that variations in the Vcom and Cs electrodes are corresponding in a set with respect to each other. Variations in the Vd in case of TFT-OFF conditions are dominated by the coupling of signals that are transmitted through three capacitance C1c, Cs, Csd as shown in FIG. 16 whereby it is desirable to set the conditions of signal coupling for these to be identical with those of the data period. Since the Vd is floating during the TFT-OFF condition, the electric potential of the Cs electrode needs to vary similarly to Vcom in order to maintain a voltage that is applied on the C1c with respect to the Vcom that is to be alternated constant (note that the DC values may be different). Therefore, variations in the Vcom and Cs electrodes are always in subordinate relations and should be considered as a set.
Japanese Unexamined Patent Publication No. 141269/1994 suggests a method in which an alternating-current voltage exceeding a threshold for liquid crystal is applied on signal lines or scanning lines in order to solve such problems during the vertical blanking period. However, the technique disclosed in this publication is merely directed to the subject of coping with display deficiencies owing to defects caused by shorting of signal lines and pixel electrodes which stand out during the vertical blanking period in which no voltage is applied, and it is not suitable to achieve the purpose of the present invention to decrease luminance differences per line.
In order to achieve the afore-mentioned purpose of the present invention, the liquid crystal display apparatus according to one embodiment of the present invention comprises a timing circuit for operating a shift register within a timing circuit during a vertical blanking period such that a common signal that has been alternated at a cycle of a Single Horizontal period is applied on counter electrodes during the vertical blanking period and such that a storage electrode signal is applied on storage electrodes having a frequency, phase and amplitude identical to those of the common signal.
The liquid crystal display apparatus comprises either an array substrate having a wiring arrangement in which gate lines concurrently serve as storage capacitances or an array substrate having a wiring arrangement in which common lines concurrently serve as storage capacitances.
The liquid crystal display apparatus comprises a timing circuit for operating a shift register within the timing circuit during a vertical blanking period such that a source signal that has been alternated at a cycle of a Single Horizontal period is applied on source lines during the vertical blanking period.
The liquid crystal display apparatus according to another embodiment of the present invention comprises a timing circuit in which a polarity inverting signal is at least one inverted during a vertical blanking period such that a variable common signal, which voltage is at least once varied during the vertical blanking period, is generated and applied on counter electrodes, and such that a variable storage electrode signal, which is varied to assume a polarity identical with the variable common signal and by an identical amplitude synchronously with the variable common signal, is generated and applied on storage electrodes.
The liquid crystal display apparatus according to still another embodiment of the present invention comprises a timing circuit in which a polarity inverting is at least one inverted during a vertical blanking period such that a signal a variable source signal, which voltage is at least once varied during the vertical blanking period, is generated, and such that the variable source signal is applied on the source lines.
The liquid crystal display apparatus according to another embodiment of the present invention comprises a timing circuit in which Gray level data are generated as data for a line following a last line, and wherein a Gray level source signal is applied on source lines during the vertical blanking period.
According to the liquid crystal display apparatus according to another embodiment of the present invention, the liquid crystal display apparatus (a) comprising a timing circuit in which a common signal and a storage electrode signal having a frequency, phase and amplitude identical to those of the common signal are generated to be of same polarity by fixing a polarity inverting signal either to H or L during each of the vertical blanking periods, and
(b) wherein a common signal of same polarity is applied on counter electrodes during each of the vertical blanking periods and the storage electrode signal is applied on storage electrodes.
According to the liquid crystal display apparatus according to another embodiment of the present invention, the liquid crystal display apparatus (a) comprising a circuit in which a common signal and storage electrode signal of a frequency, phase and amplitude identical to those of the common signal are generated by amplifying a polarity inverting signal, wherein an intermediate electric potential common signal having a potential that is between a maximum peak value and minimum peak value for an amplitude during a data period is generated by setting an amplifying rate to zero, and wherein an intermediate electric potential storage electrode signal having an intermediate electric potential that is between a maximum peak value and a minimum peak value for the amplitude during the data period synchronously with the intermediate electric potential common signal is generated, and
(b) wherein the intermediate potential common signal is applied on counter electrodes during the vertical blanking period and the intermediate potential storage electrode signal is applied on storage electrodes during the vertical blanking period.
In the driving method for a liquid crystal display apparatus according to one embodiment of the present invention, a common signal is alternated at a cycle of a Single Horizontal period and applied on counter electrodes, and a storage electrode signal having a frequency, phase and amplitude identical to those of the common signal is applied on storage electrodes during the vertical blanking period for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
In the driving method for a liquid crystal display apparatus according to another embodiment of the present invention, a source signal that has been alternated at a cycle of a Single Horizontal period is applied on source lines during a vertical blanking period for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
In the driving method for a liquid crystal display apparatus according to another embodiment of the present invention, a variable common signal, which voltage is at least once varied during a vertical blanking period, is generated and applied on counter electrodes, and a variable storage electrode signal, which is varied to assume a polarity identical with the variable common signal and by an identical amplitude synchronously with the variable common signal, is generated and applied on storage electrodes for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
In the driving method for a liquid crystal display apparatus according to still another embodiment of the present invention, a voltage for a source signal is at least once varied during a vertical blanking period and applied on source lines for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
In the driving method for a liquid crystal display apparatus according to another embodiment of the present invention, a source signal having a Gray level electric potential is applied on source lines during a vertical blanking period for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
In the driving method for a liquid crystal display apparatus according to still another embodiment of the present invention, a common signal and storage electrode signal are set to be of same polarity, wherein the common signal is applied on counter electrodes and the storage electrode signal on storage electrodes during each of the vertical blanking period for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
In the driving method for a liquid crystal display apparatus according to another embodiment of the present invention, a common signal having an intermediate electric potential that is between a maximum peak value and minimum peak value for an amplitude during a data period is applied on counter electrodes during a vertical blanking period, and a storage electrode signal having an intermediate electric potential that is between a maximum peak value and minimum peak value for the amplitude during the data period is applied, synchronous with the common signal, to storage electrodes during the vertical blanking period for decreasing effective voltage differences between odd-numbered lines and even-numbered lines and for decreasing luminance differences per gate line.
Embodiments of the present invention will now be discussed in details with reference to the accompanying drawings.